Kinases regulate many different cell proliferation, differentiation, and signalling processes by adding phosphate groups to proteins. Uncontrolled signalling has been implicated in a variety of disease conditions including, inflammation, cancer, arteriosclerosis, and psoriasis. Reversible protein phosphorylation is the main strategy for controlling activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate which drives activation is generally transferred from adenosine triphosphate molecules (ATP) to a particular protein by protein kinases and removed from that protein by protein phosphatases.
Phosphorylation occurs in response to extracellular signals (hormones, neurotransmitters, growth and differentiation factors, etc), cell cycle checkpoints, and environmental or nutritional stresses and is roughly analogous to turning on a molecular switch. When the switch goes on, the appropriate protein kinase activates a metabolic enzyme, regulatory protein, receptor, cytoskeletal protein, ion channel or pump, or transcription factor.
The kinases comprise the largest known protein group, a superfamily of enzymes with widely varied functions and specificities. They are usually named after their substrate, their regulatory molecules, or some aspect of a mutant phenotype. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The N-terminal domain, which contains subdomains I-IV, generally folds into a two-lobed structure which binds and orients the ATP (or GTP) donor molecule. The larger C terminal lobe, which contains subdomains VI A-XI, binds the protein substrate and carries out the transfer of the gamma phosphate from ATP to the hydroxyl group of a serine, threonine, or tyrosine residue. Subdomain V spans the two lobes.
The kinases may be categorized into families by the different amino acid sequences (generally between 5 and 100 residues) located on either side of, or inserted into loops of, the kinase domain. These added amino acid sequences allow the regulation of each kinase as it recognizes and interacts with its target protein. The primary structure of the kinase domains is conserved and can be further subdivided into 11 subdomains. Each of the 11 subdomains contain specific residues and motifs or patterns of amino acids that are characteristic of that subdomain and are highly conserved (Hardie G and Hanks S (1995) The Protein Kinase Facts Books, I and II, Academic Press, San Diego Calif.).
The second messenger dependent protein kinases primarily mediate the effects of second messengers such as cyclic AMP (cAMP) cyclic GMP, inositol triphosphate, phosphatidylinositol, 3,4,5-triphosphate, cyclic ADPribose, arachidonic acid and diacylglycerol. Cyclic-AMP is an intracellular mediator of hormone action in all procaryotic and animal cells that have been studied. Such hormone-induced cellular responses include thyroid hormone secretion, cortisol secretion, progesterone secretion, glycogen breakdown, bone resorption, and regulation of heart rate and force of heart muscle contraction. Cyclic AMP-dependent protein kinase (PKA) is found in all animal cells and is thought to account for the all of the effects of cyclic-AMP in most of these cells. In its inactive state, A-kinase consists of a complex of two catalytic subunits and two regulatory subunits. When each regulatory subunit has bound two molecules of cAMP, the catalytic subunit is activated and can transfer a high energy phosphate from ATP to the serine or threonine of a substrate protein. Altered PKA expression is implicated in a variety of disorders and diseases including; thyroid disorders, diabetes, atherosclerosis, and cardiovascular disease (Isselbacher K J et al (1994) Harrison's Principles of Internal Medicine, McGraw-Hill, New York City).
Protein kinase C (PKC) is a water-soluble, Ca.sup.++ -dependent kinase, commonly found in brain tissue, which moves to the plasma membrane in the presence of Ca.sup.++ ions. Approximately half of the known isoforms of PKC are activated initially by diacylglycerol and phosphatidylserine. Prolonged activation of PKC depends on continued production of diacyglycerol molecules which are formed when phospholipases cleave phosphatidylcholine. In nerve cells, PKC phosphorylates ion channels and alters the excitability of the cell membrane. In other cells, activation of PKC increases gene transcription either by triggering a protein kinase cascade which activates a regulatory element or by phosphorylating and deactivating an inhibitor of the regulatory protein. PKC activity has been specifically linked to multi-drug resistance in cancer (O'Brian C A et al (1995) Prog Clin Biol Res 391: 117-120), tumor promotion (O'Brian C A and Ward N E (1989) Cancer Metast Rev 8: 199-214) memory disorders (Saito N. et al (1994) Brain Res 656: 245-256), and auto-immune disease (Ohkusu K et al (1995) Eur J Immunol 25: 3180-3186).
A detailed understanding of kinase pathways and signal transduction is beginning to reveal some mechanisms for interceding in the progression of inflammatory illnesses and of uncontrolled cell proliferation. The novel kinases, polynucleotides which encode them, and antibodies to them satisfy a need in the art by providing a plurality of tools for studying signalling cascades in various cells and tissues, diagnosing disease and selecting inhibitors or drugs with the potential to intervene in various disorders or diseases in which altered kinase expression is implicated.